Of the estimated 30 million persons in the United States who have tried cocaine about 2 million have become dependent on the drug. Factors that determine whether cocaine use leads to dependence have not been identified clearly, but clinical evidence suggests that a prolonged period of intermittent use often precedes the development of dependence. A puzzling array of effects has been reported concerning effects of repeated cocaine exposure; sometimes sensitization is seen, sometimes tolerance. Also, effects of halting repeated exposure range from evidence for drug dependence (i.e., a disruption of behavior when the drug is withdrawn) to lingering after-effects to no evident changes. Research has revealed that behavioral factors can play an important role in determining the outcome of long-term intermittent exposure to cocaine. The research proposed is aimed primarily at trying to isolate critical behavioral/ environmental features that contribute to differences in the sequelae of repeated cocaine administration. The proposed research also investigates two other little-studied factors: dose and frequency of drugging. The maintained operant behavior of either squirrel monkeys or pigeons will be studied. Operant behavior can be considered to serve as a model of human "voluntary" behavior; it is behavior that is influenced by its former consequences. Because much human behavior involves performance of already learned activities, maintained operant behavior serves as an appropriate model. Specific types of variables to be studied include behavioral context, how well performance is maintained, effortfulness of behavior, experience between drug administration, dynamic relations between behavior and its maintaining consequences, and factors that determine whether cocaine stimulates or depresses behavior. Understanding of these variables may shed light on factors that determine vulnerability to dependence on the drug.